In recent years, the management and steering of plant growth has become increasingly specialised and closely managed. Whereas irrigation strategies were once applied field-wide with a general aim merely to compensate for lack of rainwater, plants are increasingly fed water in small groups or even individually with the aim of carefully controlling plant growth conditions.
For example, it is recognised that irrigation strategies can have a qualitative effect on the growth of plants. As is understood in the art, generative growth refers to a type of growth in which the production of flowers/fruit is encouraged, while during vegetative growth the plant a higher proportion of leaves and other green elements are produced. Generative growth is encouraged when a plant has a relative lack of water and/or nutrients, while vegetative growth is encouraged by a plentiful supply of water and/or nutrients. Vegetative growth produces the higher increase in overall biomass of the plant, while generative growth increases the proportion of the growth which contributes to the production of fruit or flowers.
It can therefore be seen that close control of plant growth conditions can be used to steer the type of growth towards a desired outcome. However, if control at this relatively fine level is to be achieved, then measurement of the plant growth conditions is critical.
One particular context in which such close control has been proposed is that of the growing of plants in mineral wool growth substrates. Such growth substrates are typically provided as a coherent plug, block, slab or mat/blanket and generally include a binder, usually an organic binder, in order to provide structural integrity to the product.
Typically, the growth process of the plant is managed in two stages: a first stage managed by a “propagator” in which the plant is grown from seed; and a second stage managed by a “grower” during which the plant is sustained and any harvest taken. For example, in the case of the tomato plant, the propagator may plant individual tomato seeds in cylindrical plugs having a thickness in the order of 25-30 mm and a radius of around 20-30 mm. After germination of the seed, the propagator places the plug within a cuboid block to allow further growth of the root system and the plant. The individual plant within the block is then nursed until a stage when it can be transferred from the propagator to the grower.
After they are received from the propagator, the grower places a number of blocks on a single slab of mineral wool to form a plant growth system. The slab of mineral wool is typically encased in a foil or other liquid impermeable layer except for openings on an upper surface for receiving the blocks with the plants and a drain hole provided on the bottom surface.
During subsequent growth of the plant, water and nutrients are provided using drippers which deliver a liquid containing water and nutrients to the system either directly to the blocks or to the slabs. The water and nutrients in the blocks and slabs is taken up by the roots of the plants and the plants grow accordingly. Water and nutrients which are not taken up by the plant either remain in the substrate system or are drained through the drain hole. Drained water and/or nutrients may be disinfected and subsequently reused if appropriate.
It is desirable to provide sensors which can be used to sense the level of water and/or nutrients of plant growth systems of this type. Suitable sensors have been proposed, and some examples are described in International patent application WO 2010/031773. FIG. 1 of this document shows a prior art water content meter having three protruding probes which are inserted into a substrate in order to measure properties such as the water content. WO 2010/031773 also describes a method for measuring capacitance of the substrate (from which the water content can be inferred) by flat plate electrodes against the surface of the substrate, rather than by inserting probes within the body of the substrate. EP0392639 describes a sensor unit with twenty inner pins having four outer pins located therearound, for measuring moisture content in a substrate. US 2011/0273196 describes a wireless environmental sensor with three or six steel pins arranged to interface with the soil at multiple points to take a single measurement representing an average property of the soil located between the pins interfacing with the soil.
While these approaches find utility, there are limits on the information that can be obtained using such techniques. In particular, it will be understood that the water content and other properties are not homogenous throughout the substrate. That is to say, there will normally be a variation in such properties throughout the substrate. Accordingly, attempts to measure water content and nutrient level from an electrical response can be too limited or variable.
For example, it may be expected that the action of gravity will cause water to settle in the bottom region of the substrate, leading to increased water content in a lower region as compared to an upper region. Other effects may depend on the location of the drain hole, for example, or the point at which irrigation is applied.
The calculation is further complicated by the properties of the substrate itself. For example, in an attempt to increase the homogeneity of water content, it has been proposed to provide substrates formed of layers having differing densities. A higher density upper layer of the substrate can increase the relative water content in that region. Nevertheless, water content remains variable throughout the substrate, making a reliable a consistent assessment of overall conditions difficult.
Closer control of plant growth conditions demands improved measurement of those conditions. There is an increasing desire to assess such conditions accurately and reliably, in such a way that plant growth strategies which operate optimally can be developed.